This invention relates to a pneumatic tire, more particularly to a tread pattern and method of designing a tread pattern for a pneumatic tire.
As a tire travels upon a surface, there occurs a movement of air, creating an audible sound. For any given tread pattern, a certain amount of energy is produced. While two different tread patterns may produce the same amount of quantum energy, the noise produced by each may have dramatically different effects on people. One tire may sound harsh and disagreeable and the other quite acceptable. The difference between the two sounds is in their frequency spectrum or degree of tonality. Tonality being the condition wherein the sound generated is dominated by a single frequency and its harmonice, i.e., a majority of the sound generated being concentrated into very small frequency range of the sound spectrum. Tonality is psychologically upsetting to a listener, creating an uneasy feeling. Additionally, besides being irritating, tonal sounds can be perceived at greater distances and require more soundproofing material than do non-tonal sounds. If a given concentration of sound energy could be spread out over a wider range of frequency, it would reduce the tonality or undesirability of the sound.
Various methods have been suggested in the past to reduce objectionable noise generated by tires by spreading the energy produced over the frequency range. This is done by modulating the event frequency or characteristic frequency by varying the design cycle length about the tire (also known as pitching). However, these methods ignore the potential tonality in the low frequency range which may result from the pitching itself. As the distance between the repeating design cycles are varied, it is possible to produce an objectionable low frequency sound, which may take the form of a low frequency flutter. This low frequency pitching tonality is readily transmitted to the vehicle and depending upon the vehicle may produce objectionable noise or vibrations.
The primary object of the invention is to effectively reduce the tonality of noise generated by a rotating apparatus having radially extending load carrying elements which produce audible noise by the utilization of a particular method of designing load carrying elements.
It is another object of the invention to modulate the noise produced by a rotating apparatus having load carrying elements by effectively spreading the event frequency energy produced over a wide frequency band, thereby reducing tonality.
A still further object of the invention is to control the selection of modulating frequencies and modulating amplitudes used to accomplish the spreading of the event frequency energy so that it does not introduce low frequency pitching tonality.
These and other objects are met and the disadvantage of the prior art are overcome by utilizing the method of the present invention as more particularly described hereinafter.